The present application relates to a storage element capable of recording information and to a storage apparatus using the storage elements.
In information apparatuses such as computers, high-operating-speed and high-density DRAMs are widely used as random access memories (RAMs).
However, the DRAM needs a complicated manufacturing process and are high in manufacturing cost, as compared with general logic circuit LSIs and signal processing LSIs used in electronic apparatuses.
In addition, the DRAM is a volatile memory which looses information when the power supply is turned OFF and, therefore, needs frequently the refreshing operation, i.e., the operation of reading the written information (data), amplifying it again, and writing it again.
In view of this, for example, the FeRAM (ferroelectric memory), the MRAM (magnetic storage element) and the like have been proposed as nonvolatile memories which do not lose information even when the power supply is turned OFF.
In the cases of these memories, the written information can be preserved for a long time, even without power supplied.
Besides, in the cases of these memories, the nonvolatile characteristic is considered to make it possible to eliminate the need for the refreshing operation and to reduce the power consumption accordingly.
However, in the above-mentioned nonvolatile memories, it becomes difficult to secure their characteristics as storage elements, attendant on the reduction in the size of the memory element constituting each memory cell.
Therefore, it is difficult to reduce the element down to the limit on a design rule basis or to the limit on a manufacturing process basis.
In view of this problem, a storage element has been proposed as a memory having a configuration suited to reductions in size.
This storage element has a construction where an ionic conductor containing a metal is sandwiched between two electrodes.
In addition, the metal contained in the ionic conductor is contained in either one of the two electrodes. This ensures that when a voltage is impressed between the two electrodes, the metal contained in one of the electrodes is diffused as ions into the ionic conductor, whereby an electrical property such as resistance and capacitance of the ionic conductor is varied.
By utilizing this characteristic, a memory device can be configured (refer to, for example, JP-A-2002-536840 and Nikkei Electronics 20.1.2003, p. 104).
Specifically, the ionic conductor is composed of a solid solution of a chalcogenide and a metal. To be more specific, the ionic conductor is composed of a material having Cu, Ag, or Zn dissolved in AsS, GeS, or GeSe, and either one of the two electrodes contains Cu, Ag, or Zn (refer to JP-A-2002-536840).
Furthermore, various nonvolatile memories using a crystalline oxide material have also been proposed. For example, there has been reported a memory based on a device having a structure in which a Cr-doped SrZrO3 crystal material is sandwiched between a lower electrode formed from SrRuO3 or Pt and an upper electrode formed from Au or Pt, and reversible resistance changes are caused by application of different-polarity voltages (refer to A. Beck et al., Appl. Phys. Lett., 77, (2000), p. 139). However, detailed principle and the like of this memory has not yet been elucidated.
However, both the above-mentioned storage element having Cu, Ag, or Zn contained in either of the upper and lower electrodes and including a GeS or GeSe amorphous chalcogenide material sandwiched between the electrodes and the above-mentioned storage element using a crystalline oxide material have a very high ON-OFF ratio of resistance, i.e., a very high ratio between the resistance in a high-low resistance state (ON resistance) and the resistance in a high-resistance state (OFF resistance), of not less than the order of 103, for example.
When a short voltage pulse is impressed on a storage element thus having a very high ON-OFF ratio of resistance, the storage element may take an intermediate value of resistance.
When the storage element takes an intermediate resistance value, the margin in data discrimination at the time of reading would be lowered.
The problem of the intermediate resistance value is considered to be generated as follows. Since the thickness of the thin film to be varied in resistance, for example, a thin film of GeS, GeSe or the like is comparatively large (for example, 10 nm or more), the intensity of the electric field upon application of a voltage is comparatively weak, so that the atoms of Cu, Ag, Zn or the like expected to move as ions would not move between predetermined positions but be trapped in their course. In addition, since the thickness of the thin film to be varied in resistance is comparatively large, the operating speed of the storage element is slow.
Further, since the intensity (magnitude) of the electric fields at the times of recording and erasing operations are low, the energy level for the ionized atoms (the atoms are transferred from an ionic state to a nonionic stat after the recording process or erasing process) after the movement to restart moving is expected to be low. As a result, it is difficult to sufficiently secure the retention (preservation) characteristic necessary to work as a nonvolatile memory.
Therefore, in the above-mentioned storage element, the storage thin film in which to record information by use of a variation in resistance thereof is desirably formed by using a material having a sufficient withstand voltage even when having a small film thickness.
Furthermore, in the condition where the storage thin film is in the low-resistance state, a current with a comparatively high current density flows therethrough, and the storage thin film is brought to a comparatively high temperature due to the Joule heat. Therefore, it is desirable to use a high melting point material for the storage thin film.
In view of the foregoing, a configuration has previously been proposed in which a high-resistance variable resistance layer is formed by use of a thin film of one of various oxides, as a storage thin film (storage layer) for recording information by use of a variation in resistance, and a layer containing Cu, Ag, or Zn is disposed in contact with the storage thin film (storage layer).
With the oxide thin film used as the storage thin film, a sufficient resistance variation can be obtained even where the film thickness is small. Therefore, the above-mentioned problems can be dissolved by reducing the film thickness and increasing the intensity of the electric field.
In the storage element in which the storage thin film (storage layer) composed of a high-resistance variable resistance layer and the layer containing Cu, Ag, or Zn are formed in contact with each other, electrode layers are provided respectively on the upper and lower sides of the laminate of these layers so that a current can be passed in the storage element.
Meanwhile, in the case where amorphous WN (tungsten nitride) is used for the electrode layer on the side of the storage thin film (storage layer), there has been the problem that, when a high voltage is impressed on the storage element in the process for transition of the storage element from the low-resistance state to the high-resistance state (the so-called erasing process), it may be impossible to bring the storage element to a sufficiently high resistance.
This is considered to be a phenomenon generated by the diffusion of the constituent element(s) of the electrode layer into the storage thin film (storage layer) due to the application of the high voltage.
Since this phenomenon is generated, it becomes impossible to secure a sufficient margin of voltage in the erasing process, or to secure a sufficient margin of the thickness of the storage thin film (storage layer) in the manufacturing process.
Thus, there is a need to provide a storage element capable of operating stably in recording information, and a storage apparatus using the storage elements.